Field of the Invention
The present invention relates to an imaging optical element and an optical scanning apparatus including the same. More particularly, the present invention relates to a multi-beam optical scanning apparatus provided with a light source including a plurality of light-emitting parts (light-emitting points) in order to achieve high-speed and high-density recording and an imaging optical element used therefor, and is suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer.
Description of the Related Art
Hitherto, a multi-beam optical scanning apparatus provided with a light source including a plurality of light-emitting parts has been widely used for a laser beam printer, a digital copying machine, or the like.
The multi-beam optical scanning apparatus is known to have such a configuration that a light flux is made incident on a deflecting surface of a deflection unit perpendicularly relative to a sub-scanning section (hereinafter referred to as “perpendicularly-incident optical system”).
In the multi-beam optical scanning apparatus provided with the perpendicularly-incident optical system, in order to uniformize intervals along the sub-scanning direction between scanning lines drawn by respective light fluxes on the same scanned surface across an entire effective scanning area, a sub-scanning magnification of an imaging optical system between the deflecting surface of the deflection unit and the scanned surface is fixed across the entire effective scanning area.
Meanwhile, in recent years, for downsizing an apparatus, an optical scanning apparatus configured to scan a plurality of scanned surfaces with one deflection unit is used. In order to scan the plurality of scanned surfaces with one deflection unit, such a configuration that the light flux is made incident on the deflecting surface of the deflection unit obliquely relative to the sub-scanning direction (hereinafter referred to as “obliquely-incident optical system”) has become frequently used.
In the multi-beam optical scanning apparatus provided with the obliquely-incident optical system, when the sub-scanning magnification of the imaging optical system is fixed across the entire effective scanning area, the intervals between the scanning lines along the sub-scanning direction are not uniform, which causes unevenness in the intervals.
In order to solve the problem, in Japanese Patent No. 4883795, there is disclosed a method of uniformizing the intervals between the scanning lines along the sub-scanning direction by causing the sub-scanning magnification of the imaging optical system to differ between a scanning starting side and a scanning ending side.
However, in the multi-beam optical scanning apparatus provided with the perpendicularly-incident optical system or the obliquely-incident optical system, when a manufacturing error such as an inclination or a shift, an assembly error, or the like occurs in optical components forming the imaging optical system or the like, a deviation from a designed value occurs in a position in which a plurality of light fluxes are imaged as a light spot on the scanned surface.
In particular, in regard to a deviation amount along the sub-scanning direction, the deviation amount differs depending on a scan position along a main scanning direction, and differs for each of the light fluxes from the plurality of light-emitting parts, and hence the unevenness in the intervals between the scanning lines along the sub-scanning direction occurs depending on the scan position along the main scanning direction, which degrades printing performance.
In order to solve the problem, in Japanese Patent No. 4418567, there is disclosed a method of optimizing a position of a main scanning stop configured to limit a light flux width in the main scanning direction and a focus distance of a collimator lens in accordance with the interval between the adjacent light-emitting parts along the main scanning direction and an fθ coefficient of the imaging optical system.
Further, in Japanese Patent Application Laid-Open No. 2013-114095, there is disclosed a method of optimizing a meridional shape of the imaging lens within the imaging optical system.
However, in the method disclosed in Japanese Patent No. 4418567, a limitation is imposed on the position of the main scanning stop and the focus distance of the collimator lens, which lowers a degree of freedom in designing the incident optical system.
Further, in the method disclosed in Japanese Patent Application Laid-Open No. 2013-114095, the meridional shape of the imaging lens within the imaging optical system is greatly bent toward an optical axis direction, and hence a limitation is imposed on an arrangement of the imaging optical system, which causes unnecessary increases in size and cost.
In order to solve the above-mentioned problem, it is desired to provide an optical scanning apparatus capable of reducing the unevenness in the intervals between a plurality of scanning lines along the sub-scanning direction due to the manufacturing error, the assembly error, or the like without lowering the degree of freedom for the incident optical system and without causing an unnecessary increase in size or cost.